Chlamydia trachomatis strains exhibit a tropism for the epithelial cells of mucosal surfaces. They recognize and bind to the host cells, are taken in by a process that strongly resembles endocytosis, and then multiply within the cell. The intracellular complex of dividing and maturing bacteria and surrounding membrane is known as an inclusion body. Under both in vitro cell culture conditions and in vivo, infection by most of the clinically important strains of C. trachomatis in inefficient; in vitro, they must be centrifuged onto the surface of host cells to achieve adequate infectivity. There is some evidence that chamydial infections are modulated in a bidirectional manner by the intracellular levels of cyclic nucleotides, with low levels of cyclic GMP stimulating infection and high levels of cyclic AMP inhibiting infection. Since cyclic AMP and cyclic GMP levels fluctuate during the cell cycle, it may be that the susceptibility of a host cell to C. trachomatis infection is limited by its position in the cell cycle. We propose to test this hypothesis. We will first establish synchronous cultures of HeLa 229 cells. Then we will inoculate the cells at various stages of the cell cycle with C. trachomatis, incubate them, and count inclusions 48 hours later. We will also measure the intracellular levels of cyclic AMP and cyclic GMP at various phases of the cell cycle. Finally, we will look for correlations between changes in cyclic nucleotide levels, cell cycle phase, and changes in susceptibility of the cells to C. trachomatis infection as measured by the number of inclusions produced.